Magnetic bearings are used for supporting shafts in various types of machinery and instruments. Passive magnetic bearings utilize only permanent magnets and have no means for electronic control. Active magnetic bearings utilize electromagnets and have associated electronic controls for controlling the current through the electromagnets and thereby the position of the shaft. Hybrid systems utilize both permanent magnets and electromagnets, with electronic controls for the latter. Active magnetic bearing systems provide the most reliable and complete form of control, and thus are the preferred magnetic bearing type for the present invention.
Magnetic bearings can be radial or axial. In active radial magnetic bearings, several electromagnets are spaced angularly around a shaft and, when energized, produce opposed magnetic forces which cause the shaft to levitate in the free space defined by the array of electromagnets. Shaft sensors detect the position of the shaft and vary the energization of the electromagnets in such a manner as to keep the shaft centered precisely on a desired axis. Axial magnetic bearings act as thrust bearings to maintain the axial position of the shaft. They are controlled in a similar fashion to radial magnetic bearings, but typically operate in conjunction with a disk supported on the shaft and act to maintain the disk in a predetermined relationship between a pair of opposed electromagnetic coils.
In a magnetic bearing system, the shaft is typically levitated before rotation, and the magnetic bearings support the shaft from that point through its entire operating range. Any loads to which the machine is subjected, such as vibrational loads and the like, are thus applied to the magnetic bearings. The control systems are adapted to compensate for varying loads to maintain the shaft in a predetermined centered position levitated within the bearings.
Because the shaft must be continually supported, the electromagnets for the bearings must be continuously energized. In some application, the amount of power consumed by the bearings is not of great consequence. Linear amplifiers which continuously drive opposed coils in a pair can be utilized, and the currents in the linear amplifiers balanced to create opposed forces which maintain the shaft levitated in a centered position between the bearings.
However, in many cases, power consumption by the magnetic bearings is a factor. For example, it is often desired to reduce power consumption in applications where only a limited amount of power is available. Furthermore, in situations where the increased heat load generated by excess power dissipated in the electromagnets of the bearings is a factor, increased efficiency translates into less heat buildup. Thus, in many applications, such as aircraft applications, the capacity of the power supply is limited, making increased efficiency desirable. In such applications, the bearings will desirably continue to operate over long periods in a reliable fashion, if not subjected to the increased heat buildup resulting from excess power dissipation. Such considerations make it desirable to drive the electromagnets with a minimum of power, concentrating the power on the forces actually needed to levitate the bearings.
The fact that the electromagnets are inductors of reasonably large inductance introduces a number of complications. In switched power supplies, such as pulse width modulated supplies, the current to the coils can be modulated. However, while it is relatively straightforward to rapidly switch an inductor on, if the inductor is switched off in a rapid fashion, the characteristic of an inductor which tends to maintain its current flow, causes the inductor, upon circuit interruption, to appear as a relatively high voltage source. In some applications, shunt diodes are typically coupled across the coil in order to prevent large transients from destroying electronic switching components, and to dissipate the excess energy in the coil. However, the energy which is dissipated is dissipated in the form of current through the coil and the shunt diode, and ultimately contributes to I.sup.2 R losses and heat buildup. Thus, not only is the energy which builds up in the coil during the on-interval wasted, but it is wasted in a way which exacerbates the problem by contributing to heat buildup.
In many applications, such as certain aircraft applications, the most readily available power supply is at a voltage level which is not necessarily optimized for the rapid turn-on/turn-off requirements desired for magnetic bearing electromagnets. It is desired to generate variable and highly controllable forces which are directly proportional to a variable control signal. The force generated by a magnetic bearing is directly proportional to the current through the bearing coil. Bandwidth (speed) of a magnetic bearing is dependent on how fast the current can be switched through the coil (di/dt). This current switching speed is governed by the equation V=L*di/dt or stated differently, the coil voltage is equal to the coil inductance times the first derivative of the coil current with respect to time. Since coil inductance is a function of the geometry of the coil and the materials of the magnets, it is relatively constant (assuming a constant magnetic bearing gap and current levels well below the saturation level of the magnetic material) and relatively independent of the coil voltage and current. Accordingly, for a given inductance (L), the current slew rate (di/dt) is dependent on the voltage applied to the coil. Stated differently, di/dt=V/L. It therefore follows that to increase the bearing bandwidth (assuming a constant L), the coil voltage must be increased. The conventional drive circuits, however, clamp the coil voltage on turn-off to a single diode drop (approximately 0.7 volts) and therefore the di/dt in the turn-off portion of the cycle is limited to 0.7/L. One of the advantages of the switching circuit of the present invention is that for the turn-off portion of the cycle, the full supply voltage is applied in reverse bias fashion across the coil. Thus, assuming a 28 volt power supply, according to the present invention, a di/dt of 28/L can be achieved. Thus, the di/dt at coil turn-off is increased by a factor of about 40 over the conventional circuit. Considering particularly the operation at relatively low DC voltages, such as 28 volts, it is expected that situations will be encountered where the tradeoffs between the inductance of the electromagnet, the forces generated, the magnetic circuit, and the desired bandwidth are insufficient at that power supply level using the conventional approach to produce the desired slew rates and bandwidth.